It has previously been proposed to operate fluorescent lamps at a frequency different from power supply frequency by rectifying alternating power supply and generating a higher frequency by using switching transistors of the same conductivity type, for example npn transistors, serially connected across the d-c power supply and connected in push-pull to a lamp circuit which includes an inductance and a capacitor to form a series resonant circuit.
Fluorescent lamp circuits of this type permit operation of the lamps at frequencies above power network frequencies--typically 50 Hz or 60 Hz, and, additionally, operation of the lamps from d-c circuits. The frequency used is usually in the low kilohertz range. Operating lamps at frequencies substantially higher than power network frequencies has the advantage of higher light efficiency. Additionally, utilizing electronic conversion networks, at a higher frequency, has the advantage that the inductive elements in the circuits will be smaller, thus have less weight and volume and cause lower heat losses in comparison to inductive elements operating at power network frequencies.
A circuit illustrating an arrangement in which a single lamp is operated at a frequency in excess of power frequency is shown in British Pat. No. 898,580, in which two transistors of similar conductivity type operate in push-pull. The push-pull transistor switch is connected over a voltage divider, consisting of two serially connected capacitors, to a 50 V d-c source. The circuit is intended for operation of a low-pressure discharge lamp with preheatable electrodes. The required starting and operating voltage is generated by a transformer which has auxiliary windings at the secondary in order to provide heater current for the electrodes. The transistors are controlled over the primary winding of the transformer, which is connected over the parallel circuit of a capacitor and a resistor with the bases of the transistors. The frequency of the oscillator, which is self-excited, is determined by the parallel resonant circuit consisting of the secondary winding and a capacitor. A typical oscillator frequency is about 1360 Hz.
Operation of a single low-pressure discharge lamp with a series resonant circuit has also been proposed, in which the series resonant circuit consists of a ballast or similar inductance serially connected with the lamp, and a capacitor connected in parallel to the lamp (see U.S. Pat. No. 3,084,283, corresponding to German Patent DE-PS No. 11 05 517). This is a typical transistor oscillator circuit in which the emitters of the two transistors are placed on a similar voltage point, the collectors are connected to the respective terminals of a transformer, and a portion of the winding thereof is connected to the lamp operating circuit. D-c operating voltage is supplied to the oscillator.
The circuits which are known and which have been described in the literature to use alternating current at a frequency above power network frequency have only moderate efficiency and, due to the comparatively long or slow switching cycles, are comparatively expensive in their construction.